Kinetic isotope effects theoretical calculations

A new study has been reported of the hydrolysis of the p-tolyldiazonium cation using a combination of carbon kinetic isotope effects, theoretical calculations, and dynamics trajectories. Overall, the results are in best accord with a process at the boundary between. S N 1 and. S N2Ar mechanisms.1 There has been a kinetic study of the thermolysis of o-carboxybenzenediazonium fluoride to give o-fluorobenzoic acid in liquid hydrogen fluoride under pressure.2... 

A study of kinetic isotope effects, theoretical calculations, and dynamic trajectories of the C(2)-C(6) (Schmittel)/ene cyclization of enyne-allenes has been reported.178 For (74), the isotope effect l=h/ l=d = 1-43 is consistent with a highly asynchronous... 

More recent theoretical work has raised questions about these conclusions, how-ever. Particularly extensive calculational treatment of the rearrangement of 54 to vinyl chloride by several research groups failed to duplicate the predictions of an atypical kinetic isotope effect. These later studies indicate that tunneling effects should indeed be greater for H-shift than for the heavier D rearrangement. Consequently, the k /ko ratio should actually decrease at higher temperatures. The discrepancy in predicted results was eventually traced to an error in the earlier calculations. Nevertheless, it... 

Table 3 collects the results obtained by different authors on the reaction of 0s04(NH3) with ethylene. Though the methods were not identical, the results are very similar, and in all cases there is a clear preference of the [3+2] over the [2+2] pathway. Additional confirmation was provided by a combined experimental and theoretical study using kinetic isotope effects (KIEs) to compare experiment and theory. Kinetic isotope effects were measured by a new NMR technique [25] and compared to values, which are available from calculated transition states. It showed that indeed only the [3+2] pathway is feasible [10]. 

Abstract This chapter describes a number of examples of kinetic isotope effects on chemical reactions of different types (simple gas phase reactions, Sn2 and E reactions in solution and in the gas phase, a and 3 secondary isotope effects, etc.). These examples are used to illustrate many aspects of the measurement, interpretation, and theoretical calculation of KIE s. The chapter concludes with an example of an harmonic semiclassical calculation of a kinetic isotope effect. 

O Neil JR (1986) Theoretical and experimental aspects of isotopic fractionation. Rev Mineral 16 1-40 Oi T (2000) Calculations of reduced partition function ratios of monomeric and dimeric boric acids and borates by the ab initio molecular orbital theory. J Nuclear Sci Tech 37 166-172 Oi T, Nomura M, Musashi M, Ossaka T, Okamoto M, Kakihana H (1989) Boron isotopic composition of some boron minerals. Geochim Cosmochim Acta 53 3189-3195 Oi T, Yanase S (2001) Calculations of reduced partition function ratios of hydrated monoborate anion by the ab initio molecular orbital theory. J Nuclear Sci Tech 38 429-432 Paneth P (2003) Chlorine kinetic isotope effects on enzymatic dehalogenations. Accounts Chem Res 36 120-126... 

The decarboxylation of 3-carboxybenzisoxazole (225 R = H, NO2) gives CO2 and (226). This reaction has been studied using 13C and 15N kinetic isotope effects.201 The isotope effects were modelled theoretically at the semiempirical and ab initio levels, but comparison of experimental and theoretical results shows that die former cannot be successfidly predicted by theory at the level of calculation employed. The kinetics of decarboxylation and deamination of DL-leucine by acidic permanganate in die presence of silver ion in moderately concentrated sulfuric acid is a two-stage process.202 The... 

A complete set of 13C kinetic isotope effects determined (by a natural abundance CMR method) for addition of lithium dibutylcuprate to cyclohexenone, in THF at —78°C, have been shown to be consistent with those calculated theoretically for ratedetermining reductive elimination from an intermediate square-planar copper complex.120 Thus, die KIE (12k/13k) = 1.020-1.026 at C(3) is indicative of substantial bonding change, and partial alkyl transfer can explain the significant low KIE = 1.011-1.016 for Ca of die butyl group. 

Today, the perepoxide intermediate proposed earlier by Sharp [35] seems to be the most popular mechanism that found support from results derived from kinetic isotope effects, stereochemical studies, trapping experiments [36-38], and theoretical calculations [39 43],... 

Experimental and theoretical studies with 13C kinetic isotope effects have revealed that the C-C bond formation is the rate-determining step of the 1,6-cuprate additions to 2-en-4-ynoates.147 The reaction intermediates have been determined and the calculated activation barrier for the transition state was found to be in good agreement with the experimental value. 

The mechanism and kinetic aspects of the retro-ene reaction of the allyl n-propyl sulfide and its deuterated derivatives have been studied using four different types of DFT methods with eight different levels of the basis sets.7 The mechanistic studies revealed that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations have indicated that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K. 

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate, catalysed by (AcO)4Rh2 and (DPTI)3Rh2(OAc), has been studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal 13C KIE was observed for the terminal acetylenic carbon, while a very small 13C KIE was detected at the internal acetylenic carbon. These isotope effects are consistent with the canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids but not with a 2 + 2-cycloaddition on a tribridged rhodium carbenoid structure.99... 

Theoretical studies of the microsolvation effect on SN2 reactions have also been reported by our coworkers and ourselves (Gonzalez-Lafont et al. 1991 Truhlar et al. 1992 Tucker and Truhlar 1990 Zhao et al. 1991b, 1992). Two approaches were used for interfacing electronic structure calculations with variational transitional state theory (VST) and tunneling calculations. We analyzed both the detailed dynamics of microsolvation and also its macroscopic consequences (rate coefficient values and kinetic isotope effects and their temperature... 

For thermal decomposition of aryl diazonium cations in water two mechanism are considered an S l mechanism involving an aryl cation and concerted frontside displacement S Ar. Kinetic, isotopic and theoretical studies delivered arguments for both mechanisms without unambiguous generalization. Application of isotope effects combined with theoretical calculations gave a new view of this problem.96 p-Tolyldiazonium cation 54 was chosen for 13C KIEs studies... 

The work is divided into several parts. Part A in Sec. II briefly sets out the relevant aspects of the RRKM formulation14 for unimolecular reactions. Rather than repeat the derivation of the equations, emphasis is placed upon the present status of the theory, and the best techniques for carrying out computations simply. In Sec. II-B, characteristics of various model hydrocarbon-type molecular species are outlined and are used in Sec. II-C for theoretical calculations that illustrate various aspects of the theory. Some related aspects of kinetic isotope effects are... 

Data for the primary kinetic isotope effect, corresponding to the replacement of the simple aldehyde or ketone by the fully tr-deuterated compound, are roughly in agreement with transition state models in which the proton is not far from being half-transferred. For example, the value observed [6.5 (Hine et al., 1972) 6.7 (Toullec and Dubois, 1974)] for fully dissociated acid-catalysed enolisation of acetone are close to the theoretical maximum value which can be calculated for half proton transfer. 

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